Present construction machines, such as wheel loaders, typically include load carrying structures, such as slab lift arms, or occasionally, a box boom lift arm, which is mounted to a frame of the machine by various connection means. The box boom lift arm is generally a hollow unitary structure made from one or more castings connected by a transversely welded midsection.
During operation of the wheel loader, it is quite common for the box boom lift arm assembly to experience a high degree of loading, some of which may be severe. Therefore, it is desirable to carry and distribute loads exerted on the box boom lift arm assembly to minimize failure of the structural elements. For improved machine performance, it is also desirable to minimize the weight of the box boom lift arm assembly while maintaining the high strength capabilities. Furthermore, it is desirable to simplify the box boom lift arm assembly manufacturing and welding processes.
Constructions of, and methods for, making load carrying structures are disclosed in the following patents. U.S. Pat. No. 3,902,295 issued to John W. Yancy on Sep. 2, 1975 and U.S. Pat. No. 4,428,173 issued to Harvey A. Knell on Jan. 31, 1984 show details of booms used on excavators. U.S. Pat. No. 4,768,917 issued to Anthony L. Garman on Sep. 6, 1988 and U.S. Pat. No. 5,152,659 issued to Toshihiko Waka on Oct. 6, 1992 show details of booms for loader type machines. In the design shown in U.S. Pat. No. 4,768,917, the boom arm is made from two hollow end castings welded together by a welded midsection. Loads on the boom arm will be experienced at the transverse welded midsection of the boom arm which may reduce overall component life. In the design shown in U.S. Pat. No. 5,152,659, the boom assembly comprises a pair of boom arms which are formed with a plurality of welded, overlapping C-channels. The overlapping of the C-channels increases the weight of the machine and complicates the welding process.
The present invention is directed to overcoming the problems as set forth above.